Metal-composite bonding

ABSTRACT

A method of bonding a graphite fiber reinforced glass matrix composite to a metal structure comprising depositing a layer of eutectic alloy metal component to the bond surfaces of the metal and composite followed by placing the thus treated bond surfaces of the metal and composite together with a layer of eutectic alloy containing such metal therebetween. The metal-bond-composite is then heated to melt the alloy and bond the composite to the metal. Articles formed by such a process are also described. The method has particular utility in bonding cooling channel containing metal bases to graphite-glass composite laser mirrors.

This is a division of application Ser. No. 215,281 filed Dec. 12, 1980now U.S. Pat. No. 4,358,512.

CROSS REFERENCE TO RELATED APPLICATIONS

Attention is directed to commonly assigned, copending U.S. patentapplications: Ser. No. 215,282, filed Dec. 12, 1980 now U.S. Pat. No.4,353,966 which disclosed a method of bonding graphite fiber reinforcedglass matrix material; and Ser. No. 215,283, filed Dec. 12, 1980 nowU.S. Pat. No. 4,350,774 which discloses a method of bonding graphitefiber reinforced glass.

DESCRIPTION

1. Technical Field

The field of art to which this invention petains is metal bonding andspecifically metal bonding of composite materials.

2. Background Art

A new class of composite materials has recently appeared in thecomposite art. This new material is graphite fiber reinforced glass.This new material, as described in commonly assigned copending U.S. Pat.Applications Ser. Nos. 54,098, filed July 2, 1979 now abandoned; 92,168,filed Nov. 7, 1979 now U.S. Pat. No. 4,263,367; and 135,375, filed Mar.28, 1980 now U.S. Pat. No. 4,265,968, has many improved properties suchas thermal conductivity, flexural strength, impact resistance, andthermal stability. These materials are generally hot pressed as anadmixture of graphite fibers of particular fiber orientation inparticulate glass. Therefore, there is generally little difficulty informing this material in any desired shape prior to hot pressing.Furthermore, it is also possible to hot press the intermediately formedcomposite into the desired design shape. However, for some applicationsit is necessary to bond various pieces of these preformed, hot pressedcomposite materials to metal pieces. Attempts at using conventionalbonding materials such as epoxy, while meeting with limited success arenot compatible with the high temperature use these composite materialsare primarily designed for and particularly adapted to, although forsome less stenuous applications, conventional adhesives could be quiteacceptable.

Particular problems occur in attempts to bond laser mirror substratesmade of such composite materials to metal parts or assemblies such asmetal heat exchangers. Because of the temperature gradients suchmetal-composite bonds undergo in operation of such mirrors, it isdifficult to get a bond material of sufficient strength and adhesion towithstand this use.

Accordingly, what is needed in this art is a bonding method for fiberreinforced composites and specifically graphite reinforced glass matrixcomposites which provides a bond which maintains the strength andthermal properties of the composite, especially when bonded to a metalsubstrate.

DISCLOSURE OF INVENTION

The present invention is directed to a method of bonding a graphitefiber reinforced glass matrix composite of high temperature strength andhigh thermal stability to a metal substrate comprising applying thinlayers of a eutectic alloy metal component to the bond surface of themetal and the bond surface of the composite followed by inserting alayer of eutectic alloy containing the deposited metal between the metalsubstrate and composite followed by raising the temperature of themetal-bond-composite to melt the alloy and bond the composite to themetal.

Another aspect of the invention includes bonded metal-composite articlesmade according to the above recited process.

The foregoing, and other features and advantages of the presentinvention, will become more apparent from the following description andaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE demonstrates a bonding process according to the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Before bonding the graphite fiber reinforced glass matrix compositepieces to the metal pieces according to the present invention, thepieces are preferably cleaned. Conventional cleaning methods can be usedsuch as ultrasonic cleaning in methanol followed by heating. The piecescan also be placed in a cathode sputtering apparatus and sputteredclean. As is conventional in the bonding art, the cleaner the surfaces,the better the bond.

The eutectic alloy metal component is next deposited both on thecomposite bond surface and the metal bond surface. While anyconventional method of depositing a thin layer of the metals may beused, e.g. plating or vapor deposition, cathode sputtering is preferred.The metals are preferably deposited in thin layers, for example, lessthan 25 microns. The eutectic alloy intermediate layer may be applied asa coating directly on either the coated metal, coated composite or both,or may be simply inserted between the metal and composite as a thinfoil, wire, ribbon, etc. If the foil form is used, it is generally from10 to 50 microns thick.

By eutectic alloy metal component is meant a free metal commonly used inadmixture with at least one other free metal which admixture has typicaleutectic melting properties, i.e. acts as a solder or braze. Forexample, if gold is used a tin-gold eutectic alloy can be used as thebonding eutectic alloy, and similarly if nickel, copper, silver, gold,chromium, nichrome, etc. are used as the eutectic alloy metal component,admixtures of these metals with their conventional eutectic alloyingmetals are also used. Note Constitution of Binary Alloys by Hanson(Second Edition, McGraw-Hill Pub. Co., 1958) for a disclosure of suchconventional alloys.

After application of all of the metal layers and eutectic alloy themetal component, bonding alloy, and composite are laid one on top of theother and subjected to a slight pressing, e.g. 2 to 15 psi (1.4×10⁴ to10.3×10⁴ NT/M²) and the composite-bond-metal heated to a temperaturesufficient to melt the bonding alloy. While this heating may take placein air, improved bonding results if the heating is performed in anon-oxidizing or even reducing atmosphere such as argon or hydrogen.Similarly, while the heating may be performed at atmospheric pressure,it may be performed under vaccum as well. For example, a piece ofmolybdenum metal and graphite fiber reinforced glass matrix material areground flat to substantially matching contours and cleaned. Gold layersapproximately one micron thick are then deposited by cathode sputteringonto the cleaned and bond surfaces of both the metal and the composite.A gold-tin eutectic alloy in foil form approximately 25 microns thick isthen inserted between the coated metal and composite pieces to bebonded. The pieces are stacked, bond surfaces together as shown in theFIGURE, where 1 is the graphite fiber reinforced glass matrix composite,3 molybdenum metal, 2 and 4 the deposited gold layers, and 5 the 80% byweight gold, 20% by weight tin eutectic alloy foil. A pressure ofapproximately 2 to 15 psi (1.4×10⁴ to 10.3×10⁴ NT/M²) is then applied tothe composite-bond-metal material and the temperature raised to about280° C. The samples were tested in a tensile test with a bond area of0.3 in.² (1.94 cm²) and resulting bonded material was found to have abond strength of at least 2800 psi (1.9×10⁷ NT/M²).

While this bonding process has been described for graphite fiberreinforced glass composites, the process according to the presentinvention may be used for any fiber reinforced glass or glass-ceramic orceramic composite. Specifically, the process is designed for bondingthose composites disclosed in commonly assigned copending U.S. patentapplications Ser. Nos. 54,098, filed July 2, 1979; 92,168, filed Nov. 7,1979; and 135,375, filed Mar. 28, 1980, the disclosures of which areincorporated by reference.

As described in the above patent applications, while any graphite fiberwith the requisite high strength and good modulus of elasticity can beused in the composites of this invention, such as Hercules HMS graphitefiber, Celanese GY-70 (formerly DG102) graphite fibers are particularlysuitable. This fiber consists of 384 fibers/tow and has an oxidizedfinish. It is 8 microns in diameter, has a modulus of elasticity of 531GPa (77×10⁶ psi). It has a tensile strength of 1724 MPa (250×10³ psi)and a density of 1.96 gm/cm³. The fiber is used at about 40% to 70% byvolume based on the graphite-glass composite and preferably at about 60%by volume. Also, as described in these applications, the glass used inthe composites of the present invention is particularly selected to havea very low coefficient of thermal expansion preferably matched closely,but not equal to that of the graphite fibers used since the graphite hasa highly negative axial coefficient of thermal expansion, and the glasshas a positive but small coefficient of thermal expansion. Particularlysuitable for the purposes of this invention is a borosilicate glass(Corning Glass Works 7740) with an anneal point of 500° C., a softeningpoint of 821° C., a liquidus temperature of 1017° C., a density of 2.23grams per cubic centimeter, an index of refraction of 1.474, adielectric constant of 4.6, a coefficient of linear expansion of 32.5cm/cm°C.×10⁻⁷ and a modulus of elasticity of 9.1×10⁶ psi (6.3×10⁷NT/M²). The particle size of the glass should be such that at least 90%passes through a 36.0 mesh screen.

The main advantage of the bonding material of the present invention isthat it allows fabrication and flexibility in design of the graphitereinforced glass composites described in the above recited patentapplications without sacrifice in the improved strength, impactresistance and thermal stability of the composite material. For example,the resulting bond between composite pieces has stability over a widerange of temperature conditions. The bond strengths in excess of 2800pounds per square inch (1.9×10⁷ NT/M²) are significant for any bondingmaterial excluding the difficult bondability of the graphite fiberreinforced glass composites of the present invention.

One area of particularly important utility for this method is withcomposite laser mirrors as described in copending U.S. patentapplication Ser. No. 54,098, filed July 2, 1979, the disclosure of whichis incorporated by reference. A conventional use for these mirrors is inconjunction with a metal face plate or heat exchanger such as amolybdenum block containing cooling channels. It has been difficult tofind bonding agents which will hold such a metal-composite mirrortogether without degrading the properties of the composite. However,utilizing the process according to the present invention, a molybdenummetal faced graphite fiber reinforced glass composite mirror can be madewithout excellent bond strength.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail thereof may be madewithout departing from the spirit and scope of the claimed invention.

I claim:
 1. A method of bonding a high strength, thermally stable,graphite fiber reinforced glass matrix composite to a metal surface, theglass matrix having a low coefficient of thermal expansion and thegraphite fiber having a highly negative axial coefficient of thermalexpansion comprising depositing a thin layer of a eutectic alloy metalcomponent to the bond surfaces of the composite and the metal, insertinga thin layer of eutectic alloy containing the metal component betweenthe bond surfaces, placing the metal and composite bond surfacestogether with the eutectic alloy in between and raising the temperatureof the metal-alloy-composite to a temperature sufficient to melt thealloy and bond the composite.
 2. The method of claim 1 wherein the metalcomponent layers are deposited in thicknesses up to 25 microns.
 3. Themethod of claim 1 wherein the bond produced has a strength of at least2800 psi (1.9×10⁷ NT/M²).
 4. The method of claim 1 wherein the graphitefibers have a modulus of elasticity of at least 531 GPa, a tensilestrength of at least 1724 MPa and a density of about 1.96 gm/cm³.
 5. Themethod of claim 1 wherein the composite contains 40% to 70% by volumegraphite fibers having orientation in the composite of 0° and 90°; 0°,45° and 90°; 0°, 60° and 120°; are discontinuous; or have fiberorientation substantially perpendicular to the bond.
 6. The method ofclaim 1 wherein the metal surface is molybdenum.
 7. The method of claim1 wherein the eutectic alloy metal component is gold and the eutecticalloy is a gold-tin mixture.